Generally, a non-oriented electrical steel sheet is an important part that is necessary for converting electric energy into mechanical energy in electric devices. In order to reduce the consumption of energy, it is necessary to decrease iron loss and increase magnetic flux density. Particularly, when the magnetic flux density is increased, power loss in an electric device can be decreased, thereby enabling the miniaturization of the electric device.
This non-oriented electrical steel sheet is used as an iron core material in rotating devices such as motors, generators and the like and in stationary devices such as small-sized transformers, and is the most important part of these electric devices. An iron core is used to increase the strength of a magnetic field when the magnetic field is formed by supplying electric current. In this case, when the magnetic property of the non-oriented electrical steel sheet is excellent, the efficiency of a motor is high and the consumption of electricity can be reduced.
Recently, an electrical steel sheet for a motor for moving an electric car has received a lot of interest because the most important material used for the motor is a non-oriented electrical steel sheet.
Magnetic properties of the non-oriented electrical steel sheet include iron loss and magnetic flux density. Iron loss is loss occurring when a magnetic field is applied, and magnetic flux density is the amount of work that can be performed at that time, that is, rotation power in a motor. Accordingly, it is preferred that iron loss be low, and it is required that magnetic flux density be high.
In this case, the iron loss can be decreased by decreasing the thickness of the electrical steel sheet or adding alloy elements in large quantities. However, it is difficult to improve both the iron loss and the magnetic flux density.
In order to manufacture a material having a low iron loss and a high magnetic flux density, clean steel having a small amount of impurities must be manufactured, or steel having improved magnetic properties must be manufactured by adding additional elements. However, there are problems in that, in the former, the manufacturing cost is increased due to the additional processes in the process of manufacturing the clean steel, and, in the latter, the cost of additional elements is increased due to the addition of the additional elements.
In this non-oriented electrical steel sheet, factors having an influence on magnetic properties include additive components and impurity components, and crystal grain sizes and texture in material properties.
In this case, with the increase of the crystal grain sizes, the iron loss is decreased. However, since the magnetic properties are deteriorated when textures that easily magnetize the steel sheets are not formed in spite of largely grown grain size of steel sheets, it is inferred that the texture is more important than the crystal grain size. In the texture, (200) plane, including crystal orientations in which the steel sheets are easily magnetized in a direction parallel to the surface of rolled sheets, is preferable. Further, it is preferred that there be few (111) plane or (211) plane.
In conventional technologies for this non-oriented electrical steel sheet, in Japanese Unexamined Patent Application Publication No. 1996-283803, since the amount of Mn is limited to 0.1% or less, it is difficult to coarsen MnS, which is a fine precipitate.
Further, in Japanese Unexamined Patent Application Publication No. 1996-283853, the amount of impurities is limited, if possible. However, since various kinds of impurities are present, it is difficult to control the amount of the impurities, and the relationship of the impurities to the manufacturing process conditions is unclear.
Further, in Japanese Unexamined Patent Application Publication No. Hei11-222653, it is described that, as the amount of sulfur (S) etc. used as impurities is decreased, magnetic properties are increased, but the amount of sulfur (S) does not relate to the manufacturing process conditions.
Further, U.S. Pat. No. 6,139,650 discloses a non-oriented electromagnetic steel sheet and method for manufacturing the same, in which the amount of sulfur (S) is decreased to 0.001% or less, and elements such as Sn, Sb and the like are additionally added. Here, although the amount of impurities is decreased and additional elements are added, the relationship of other components to the manufacturing process conditions is not described.